Japanese Patent Application Laid-Open (kokai) No. 2-207475 describes “a spark plug for an engine configured such that a center electrode and a ground electrode are electrically insulated from each other by means of an insulator, and characterized in that a main insulator formed from silicon nitride (Si3N4) or mullite (3Al2O3.2SiO2) surrounds the center electrode, and an insulator formed from aluminum oxide (Al2O3) is provided on an inner circumferential surface of the ground electrode” (see claim 1 of Japanese Patent Application Laid-Open (kokai) No. 2-207475).
Japanese Patent Application Laid-Open (kokai) No. 2001-2465 describes “an insulator for a spark plug which contains alumina (Al2O3) as a main component and at least two components of a trinary system containing a silicon component, a calcium component, and a magnesium component, characterized by having at least a crystalline phase of mullite (Al6Si2O13) and being formed from an alumina-based sintered body having a relative density of 95% or higher” (see claim 1 of Japanese Patent Application Laid-Open (kokai) No. 2001-2465).
According to the invention described in Japanese Patent Application Laid-Open (kokai) No. 2-207475, the spark plug has high resistance to thermal shock and sufficient dielectric breakdown voltage (see the section “Object of the Invention” in Japanese Patent Application Laid-Open (kokai) No. 2-207475).
According to the invention described in Japanese Patent Application Laid-Open (kokai) No. 2001-2465, the insulator exhibits excellent dielectric strength in a temperature range of room temperature to high temperature and is densified. (See paragraph 0010 of Japanese Patent Application Laid-Open (kokai) No. 2001-2465).
The presence of a crystalline phase in a sintered body improves mechanical strength. By contrast, the presence of aggregates stemming from a failure to achieve densification lowers mechanical strength. Since the insulator for a spark plug is formed through liquid phase sintering, usually, a sintering aid is added in an amount of about 2 mass % to 10 mass %. However, since the size of aggregates is large, a portion of sintering aid that remains unmolten and compounds produced through reaction remain in the form of a non-vitreous substance. Since these residual substances are not in crystalline phase, the insulator for a spark plug formed through liquid-phase sintering involves a problem of deterioration in mechanical strength.